Broadband automatic tracking antenna



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BROADBAND AUTOMATIC TRACKING ANTENNA Filed July 2, 1963 3 Sheets-Sheet 1 INVENTORS VERNON C. SUNDBERG KENNETH L. WALTON ATTORN E Y A g- 66 v. c. SUNDBERG ETAL 3,264,548

BROADBAND AUTOMATIC TRACKING ANTENNA .6 Sheets-Sheet 2 Filed July 2, 1963 2, 1966 v. c. SUNDBERG ETAL 3,264,648

BROADBAND AUTOMATIC TRACKING ANTENNA Filed July 2, 1963 3 Sheets-Sheet 5 I l0 a: U1 o 0.

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3O 3O 3O BEARING-DEGREES BEARING-DEGREES Fl [3 El .0 1: ,INVENTORS VERNON C. SUNDBERG .5 .6 .8 L0 L5 2.0 3.0 4.0 EN E WALTON ATTORNEY United States Patent 3 264,648 BRUADBAND AUTOMATIC TRACKING ANTENNA Vernon C. Sundberg, Santa Clara, and Kenneth L. Walton,

unnyvale, Calif, assignors to Sylvania Electric Prodnets Inc, a corporation of Delaware Filed July 2, 1963, Ser. No. 292,298 1 Claim. (Cl. 343-454) This invention relates to antennas and in particular to a broadband tracking antenna. Broadband as used herein means three or more octaves of frequency and therefore the ratio of the highest to the lowest frequency of the band is at least eight to one.

The desirability of a high gain microwave tracklng antenna possessing frequency independent pattern characteristics has long been recognized, but no satisfactory antenna with these characteristics has been devised. Such antennas are used for automatic tracking of satellite-borne beacons which require that the antenna system be capable of monitoring a wide band of frequencies while tracklng at one frequency within that band. Tracking antennas ordinarily employed for these purposes invariably involve design compromises due to the frequency dependent character of the pattern, the pattern width at any reference level varying approximately inversely as the frequency. This invention concerns a tracking antenna of the conical scanning type which can produce a beamwidth pattern with substantially constant crossover power level and gain over three octaves of frequency at microwave frequencles.

An object of the invention is the provision of a tracking antenna capable of operating over at least three octaves of frequency without any adjustment of its components.

Another object is the provision of a broadband conical scanning automatic tracking antenna having a substantially constant beam crossover level for at least three octaves of frequency.

These and other objects of the invention will become apparent from the following description of a preferred embodiment thereof, reference being had to the accompanying drawings in which:

FIGURE 1 is a perspective view of a tracking antenna system embodying the invention, showing a Waveguide horn as a primary feed device and a paraboloidal reflector as a secondary or collimating aperture;

FIGURE 2 is a schematic elevation of the system showing the illumination of the paraboloidal reflector by the primary feed horn over the full frequency range of the antenna system;

FIGURE 3 is a schematic front view of the reflector illustrating the pattern of illumination of the reflector at the low and high frequency ends of the band by the horn as the latter nutates at the focal point of the reflector;

FIGURE 4 is a side elevation partly broken away of a double-ridged waveguide horn which is the primary feed element of the antenna system;

FIGURE 5 is a transverse section taken on line 55 of FIGURE 4;

FIGURE 6 is four plots of actual radiation test patterns of the antenna of FIGURE 1 over three octaves of frequency and showing the secondary beam crossover in the horizontal plane; and,

FIGURE 7 is a plot of secondary beam crossover level against frequency.

The invention in its fundamental form consists of an equiphase primary aperture axially spaced from and nutated relative to a secondary or collimating aperture so that substantially all of the secondary aperture is of fectively illuminated by the nutating primary aperture at the lowest frequency in the operating bandwidth of the antenna system. As the frequency is increased, the angular width of the beam from the primary aperture which illuminates the secondary aperture decreases proportion- 3,264,648 Patented August 2, 1966 ally with the decrease in wavelength such that the effective dimensions of the second aperture remain essentially constant in terms of wavelength. The primary aperture in its preferred form is a double ridged waveguide horn with a dielectric lens, and the secondary aperture is a paraboloidal reflector. The horn is nutated at the focal point of the paraboloidal reflector about an axis angularly related to the focal axis of the reflector so that the latter is asymmetrically illuminated.

Referring now to the drawings, a preferred embodiment of the invention is shown in FIGURE 1 and comprises a paraboloidal reflector 10 and a primary feed device 12 mounted at the focal point of the reflector on a supporting boom 14. The focal axis F of reflector 10 is located above the boom 14 as shown. The beam axis B of the primary feed device 12 intersects the focal axis F of the reflector at an angle 5 as shown in FIGURE 1 so that electromagnetic wave energy is radiated by the feed device 12 toward portions of the reflector lying generally above the intersection G of focal axis F with reflector 10. Feed device 12 is mechanically coupled to a nutating mechanism 16 which causes the beam of radiated energy from the feed device to nutate as it illuminates the reflector 10 as described in greater detail below.

Reflector 10 is supported by rigid framework 18 on a pedestal 19 for tracking movements in azimuth and elevation. The boom 14 is mechanically rigidly secured to framework 18 below the point G of the reflector and is itself a bridge-like structure which supports the primary feed device 12 and nutating mechanism 16 with a minimum of vibration.

The illumination of reflector 10 by feed device 12 is illustrated schematically in FIGURES 2 and 3. The frequency independent operation of the antenna system results from the condition in which the reflector 10 receives substantially all of the radiation of the primary feed de vice over the operating bandwidth of the system. This places a limit on the low frequency end of the band since the beamwidth of the primary feed device lobe is proportional to the wavelength as expressed approximately by the relation where (2 is the total angular width of the primary feed lobe at the low frequency end of the band, D is the diameter of reflector 10, f is the focal length of the reflector, M is the wavelength at the low frequency end of the band, and d is the width of the primary feed device aperture, see also FIGURE 4. In practice the angular width 04 of the feed lobe at the low frequency limit is such that the edge illumination of the reflector 10 is sufficient (in the order of 10 db) to provide the desired amplitude distribution. As the primary feed device nutates, the primary feed lobe moves over the entire surface of reflector 10 to produce the desired conical scan for automatic tracking purposes.

The pattern of illumination of the reflector 10 by the nutating primary feed lobe is shown schematically in 'FIG- 'U-RE 3. The feed device 12 is shown as being vertically polarized in this instance with the arrows indicating the direction of the electric field vector E. The polarization of the feed device is linear and is adjustable in angle. The four circles M M M and M in that order, represent approximately the areas of the re'fleotor successively illuminated by the primary feed device 12 in the low frequency range of the system as the feed device nutates in a clockwise direction as viewed. The smaller circles N N N and N are corresponding areas of reflector illumination by the nutating primary feed device at the upper end of the frequency range.

Limitation on illumination of reflector 10 at the low frequency end of the operating band is expressed in terms of the system parameters by:

1 D N X1 a-s1n wfif g -sm 1 K- The constant K depends upon the illumination function of the primary feed aperture and also upon the level of illumination intensity chosen for the edge of the reflector at this wavelength, M. It the sfirst nulls (which define the major lobe) of the primary rfeed pattern are set on the edge of reflector 10 and the illumination across the aperture of the primary feed device 12 is a cosine function as explained in Microwave Antenna Theory and Design, by Silver, page 187 (McGraw-Hill, 1949), K will equal 1.5. This is unnecessarily restrictive, however, and some spillover of the illumination of the aperture of reflector 10 is allowed. Assuming that edge illumination of the reflector 10 is allowed to rise to the 10 db point on the primary feed major lobe at the long wavelength limit, then with cosine illumination of the aperture of primary feed device 12, K will be approximately unity.

Solving Equation 2 for A Dd Dd It might be expected that an upper frequency limit would exist because at sufficiently high operation frequencies the primary feed device no longer appears as a point source at the focal point of reflector 10. This limit may not occur, however, as can be shown by consideration of the form of the radiation pattern of the reflector 10 as the operating wavelength approaches zero. In the optical limit, the energy distribution at the primary feed device would be projected to infinity by reflector 10 and the beamwidth of the collimating aperture lobe between nulls, called the secondary beamwidth, would then be d/f.

Near the low frequency limit, the primary feed device main lobe illuminates substantially all of the reflector aperture as suggested by the large circles M to M in FIGURE 3, and the secondary beamwidth is 3ND. However, under these conditions, as was shown earlier by Equation 3 With this value of A a secondary null beamwidth equals which is equal to the optical limit case, or where 0. It has been shown that for the cosine primary linear aperture distribution, the secondary pattern is essentially the same for an operating wavelength near the longer limit as it is for wavelengths approaching zero.

The primary feed device 12 comprises a waveguide horn having ridges 23 and 24 projecting inwardly from the sides thereof and having a dielectric lens 25 disposed in its aperture 26. 'Ridges 23 and 24 are tapered within the flared section of the horn to zero height at the horn aperture in order to smoothly transform the impedance of the ridged waveguide to the impedance of free space. The physical dimensions of horn 22 are selected so as to depress deleterious high order modes over a bandwidth in excess of 10 to 1. By providing the solid dielectric lens in the horn aperture for correction of phase error, the longitudinal dimension of the horn is desirably shortened. This reduces the dynamic loading at the end of the supporting boom and minimizes vibration error.

The horn 22 is supported and nutated by nutating mechanism 16 which may be and preferably is of the type described in copending application S.N. 303,322, filed August 20, 1963, assigned to the assignee of this application. Alternatively, other nutating mechanisms such as that described in Volume 26 of Radiation Laboratory Series, page 64 et seq. (McGraw-Hill, 1948), may be employed. A coaxial line 28 connected to the horn 22 through the nutating mechanism 1'6 carries microwave energy to and from the horn.

A conical scanning automatic tracking antenna system embodying this invention was operated successfully and produced radiation patterns shown in FIGURE 6. It will be noted from FIGURES 6 and 7 that maximum crossover level variation over three octaves of frequency is from 05 db to 5.0 db and that the Nariation at the high and low frequency limits is about 3.0 db. The parameters of this antenna system are:

Diameter of reflector 10 (dimension D) feet 30 Focal length 1 do 17.5 Primary Feed Horn 12:

Length 1 do 3 Aperture 26:

Height (E plane) inches 13 /2 WidthiQH plane) do 18 Beam angle :1 at high frequency {10 db) "degrees" Beam angle m at low frequency (10 db) do 25 'Nutation velocity r.p.m 900 Frequency range gc 0.5 to 4.0 Maximum change in beam crossover level over frequency range db 4.5 Gain of system:

Low frequency db 30 High frequency db 45 Changes, modifications and improvements to the above described embodiment of the invention may be made by those skilled in the art without departing from the spirit or scope of the invention. The appended claim defines the novel features of the invention.

What is claimed is:

A broadband tracking antenna system having an operating frequency range with a lower limit f and an upper limit f comprising a paraboloidal reflector having a focal point, and a focal axis,

a pedestal,

means for supporting said reflector on said pedestal for tracking movements in azimuth and elevation,

a boom secured to and projecting from said reflector below said focal axis,

a nutation mechanism supported on said boom adjacent to the focal point of the reflector, a primary microwave feed device at the focal point of the reflector and coupled to said nutation mechanism,

said feed device comprising a waveguide horn adapted to propagate linearly polarized electromagnetic waves toward said reflector as a primary beam,

said horn having an aperture and having double internal transverse ridges extending in the direction of the electric field of said waves and tapering to zero height at said aperture, and

a dielectric lens in said aperture adapted to equalize the phase of waves propagating therethrough,

the aperture of the horn and said reflector having transverse dimensions of predetermined relative magni tude such that the nutating major lobe of the horn at frequency f subtends the entire area of the reflector aperture and the nutating major lobe of the horn at frequency f subtends an area of the aperture substantially less than the entire aperture area,

said horn being operatively supported by said nutating mechanism relative to said reflector such that said primary beam is nutated about an axis angularly related to said focal axis.

References Cited by the Examiner UNITED STATES PATENTS King 343-762 Gluyas 343-840 King 343-761 vPlummcr et al 343-761 Foley et a1. 343-772 Brown et a1 343-757 Bohm et a1. 343- 840 6 FOREIGN PATENTS 601,280 3/1948 Great Britain. 75 3,03 8 7/ 1956 Great Britain. 800,293 8/ 195 8 Great Britain.

OTHER REFERENCES Silver, Microwave Antenna Theory and Design, Radiation Laboratory Series, vol. 12, pages 35 8 to 3 65 relied on.

10 ELI LIEBERMAN, Acting Primary Examiner.

R. F. HUNT, JR., Assistant Examiner. 

